The invention relates to a servo system, particularly a servo system for trip recorders for measuring, recording, and displaying the speed, whether the rotational speed (rpm) or linear speed (miles per hour), of a vehicle. The direct current signal proportional to the speed and the direct current signal proportional to the indicated speed are fed back to a comparison junction that compares these two signals. The junction is connected to the input means of the operational amplifier. The amplification factor for the difference voltage is controlled by a feedback path. The amplifier controls the direction of rotation of the servo motor that operates display and recording instruments.
The purpose of a trip recorder is to measure, record, and to display data that show the operation of the vehicle. Of particular importance, is the measurement of the speed of the vehicle and rotational speed of the engine. A trip recorder also indicates the distance travelled and the times that the driver is at the wheel and not at the wheel in separate time diagrams.
The trip recorder occupies a special place among comparable recorders. Since it is built into a vehicle, it is subject to voltage changes of the vehicle battery of as much as 40% of the regular voltage and to temperature variations between +70.degree. C. and -30.degree. C. Aside from this, it is shaken sometimes violently, by the road surface and by acceleration, the latter chiefly in the direction of forward movement. These quite unfavorable operating conditions for a measuring instrument are diametrically opposed to the requirement of an accurately recorded trip diagram. The recording on the diagram disk is used more and more today as evidence for determining guilt in an accident, for statistical purposes, and even as a basis for determining the salary of the driver.
The eddy current system is preferred today for most trip recorders, particularly because it is robust, but also because it is simple and cheap to manufacture. The nonlinear relationship between rpm and the angle of rotation and the high driving torque of this system must both be tolerated. The eddy current system, however, must be driven through a mechanical connection, such as a flexible shaft or a universal joint shaft.
It has been suggested to drive the eddy current system directly from an electric motor that is located at the trip recorder and which turns at the rpm that is to be measured. This scheme, which electrically transmits the quantity to be measured, avoids the well-known disadvantages of shafts: faulty operation and breakage at low temperatures, as well as a tendency to torsional vibration because of the usually unavoidable bends in the shaft necessitated by space limitations. This solution requires, however, a great deal of irreplaceable electrical energy in the vehicle, and is not only costly but also technically round about.
If the difficulties of mechanical drive are to be avoided, it would be more logical to employ with the electric transmission of the value to be measured an electric circuit for suitably preparing, or transforming, the value to be measured. The electric transmission can be in the form of pulses. Experiments along these lines have shown that only a measuring circuit using a balancing arrangement meets the requirements. In other words, a servo system having a comparision junction (mixing point), an amplifier, and servo motor.
It should be remarked, speaking generally of electric measuring systems, that in comparison with the electro-mechanical eddy current system with its unavoidably large masses, there are appreciably more and simpler opportunities to reduce dynamic error. A balancing measuring arrangement continuously compares the actual value with the desired value and, if the two values are not equal, the arrangement is adjusted until the difference between the two is zero. The special characteristics of a balancing arrangement (accuracy relatively immuned to variations in battery voltage and in amplifier gain, a high attainable adjusting power, and a nearly infinite input impedance at balance) can be advantageously used, particularly with apparatus that is subject to severe operating conditions.
As the following considerations will show, great demands are placed on the speed and accuracy of the servo system. The trip recorder must display and record over a wide range of rpms. Moreover, the vehicle speed often changes suddenly when shifting from one gear to another during fast acceleration or sharp braking. The recorded diagram, nevertheless, must show the true changes in the measured values if it is not to be useless as a record.
The electric servo systems previously suggested do not satisfy these demands. Even expensive and complicated feedback amplifiers, fundamentally differential amplifiers (German Pat. No. 1,229,325, also describes a magnetic amplifier), have too long a response time and are driven by the ever-present residual ripple in the signal of the desired value to cause the servo motor to over shoot. Moreover, the rated speed of the servo motor is insufficient, chiefly because of too little amplification in the output stage. In many instances, the output stage requires a high idling current, a fact that is intolerable in a motor vehicle. Since the servo motor tends to over shoot, because of insufficient balancing torque particularly when the measured value changes suddenly, the over shoot is fed back to the comparison junction, and the measured value is not accurately recorded. The motor's tendency to over shoot is encouraged by the inertia of the vibration prone parts of the recorder and display instruments that are connected to the motor.
Consequently, the accuracy of the displayed value over the required range is too unstable. Even the display itself in certain ranges, particularly at low rpms, was unstable. The chief disadvantage of previously tested servo systems was, however, that their susceptability to trouble made it questionable as to whether a stable controller action could be reproduced in system after system, as would be required for mass production.